1. Field of the Invention
The present invention relates to a coated optical fiber unit comprising at least two fibers which are coated together with a bundling coating layer, and each of which has a protective coating layer around an optical fiber. In particular, the present invention relates to an improvement to a resin of a bundling coating layer and a coloring layer.
2. Description of the Related Art
Since an optical glass fiber to be used as an optical transmission medium (hereinafter referred to as "optical fiber") has a diameter of 150 .mu.m or less usually, and is made of a fragile material, namely glass, its surface tends to be easily flawed during its production or cable assembling, or storage. Further, the stress is concentrated on the flawed part so that the optical fiber is easily broken when an external force is applied thereon.
Since it is very difficult to use such an optical fiber as an optical transmission medium, the surface of optical fiber is coated with a resin just after production make the optical fiber suitable and to be worthy for long term use. The coated optical fiber is practically used.
Examples of the conventionally used resinous coating material are thermosetting resins (e.g. silicone resin, epoxy resin, urethane resin, etc.), actinic ray (UV light)-curable resins (e.g. polyepoxy acrylate, polyurethane acrylate, polyester acrylate, etc.), and the like. Since these materials are organic materials, it is known that these materials suffer from denaturation such as oxidation, or deterioration. To prevent denaturation, they are treated to suppress the generation of hydrogen gas (H.sub.2), as disclosed in Japanese Patent KOKAI Publication No. 113711/1989.
To improve the handling easiness and a density of the optical fibers, a coated optical fiber unit in which two or more coated optical fibers are coated together with a bundling coating layer has been used.
When the single optical fiber of the coated optical fiber unit is fusion connected with an optical fiber, with a single optical fiber of another coated optical fiber unit, or with a connector, an end part of the bundling coating resin should be removed to separate the single optical fibers. Each coated optical fiber has a coloring layer around the protective coating layer to identify each optical fiber when it is separated as described above.
When a bond strength between the bundling coating resin and the coloring layer is larger than that between the coloring layer and the protective coating layer, the coloring layer is removed when the bundling protective resin is removed. Consequently the respective optical fiber may not be identifiable.
To solve such problem, Japanese Patent KOKAI Publication Nos. 281109/1988 and 57814/1992 disclose either treating the bundling coating resin or the coloring layer to suppress bonding, whereby the bond strength between the bundling coating resin and the coloring layer is decreased.
However, to decrease the bond strength by the above treatment for suppressing bonding, a releasing agent should be added to the resin or the coloring layer at a high concentration. If the releasing agent is added at a high concentration, it will cause various problems, since it is a low molecular compound and easily moves in the bundling coating layer, coloring layer and protective coating layer which are made of polymers.
When the releasing agent migrates to the inner resin of the coloring layer, the resin swells until causing the resin layer to crack. When the releasing agent migrates to an interface between the optical fiber and the protective coating layer, a part of the protective layer is peeled off from the optical fiber, and the releasing agent which is accumulated between the protective coating layer and the optical fiber generates uneven stress on the optical fiber, which may increase transmission loss.
When the releasing agent migrates onto the surface of the bundling coating layer, the surface of the bundling coating resin becomes sticky so that the handling property of the unit is deteriorated. In addition, the releasing agent aggregates to cause phase separation in the resin.
The bonding between the bundling coating layer and the coloring layer is achieved because the surfaces of the bundling coating resin and the coloring layer are adhesion active physically (e.g. unevenness) or chemically (e.g., the presence of functional groups which form a hydrogen bond).
The above treatment for suppressing bonding is aimed to make either the bundling coating layer surface or the coloring layer surface adhesion inactive. Since the resin of the other layer is still adhesion active on its surface in the above treatment, the releasing agent should be added to one resin in a very high concentration such that the surface of the other resin is made adhesion inactive, so that the bonding is suppressed by the addition of the releasing agent. Otherwise, no effect is obtained.